1. Field of the Invention
The present invention relates an optical semiconductor device and a method of controlling the same.
2. Description of the Related Art
A wavelength-tunable semiconductor laser lasing some wavelength with a single element is being developed in many organizations, along with a prevalence of Wavelength Division Multiplexing (WDM) communication using an optical fiber. A wavelength-tunable semiconductor laser proposed before now is roughly classified into two types. One of the two types has a Semiconductor Optical Amplifier (SOA) in an external resonator, and controls a lasing wavelength with use of a wavelength selectable portion included in the external resonator. The other of the two types has a structure in which a wavelength-selectable resonator is built in a semiconductor element having a gain with respect to the lasing.
A representative wavelength-tunable semiconductor laser, in which the resonator is built in the semiconductor element, is a laser having a Sampled Grating Distributed Reflector (SG-DR) waveguide. U.S. Pat. No. 6,590,924 (hereinafter referred to as Document 1) discloses a laser using a vernier effect. In the laser, a first SG-DR waveguides is combined to a first side of a waveguide having a gain for lasing, and a second SG-DR waveguide having a different longitudinal mode interval from the first SG-DR waveguide is combined to a second side of the waveguide. If a reflection peak wavelength of the first SG-DR waveguide and a reflection peak wavelength of a longitudinal mode of the second SG-DR waveguide are changed according to temperature and current, the laser oscillates at a wavelength where each of the peak wavelength corresponds to each other.
However, the laser disclosed in Document 1 may oscillate at a wavelength other than a desirable wavelength because of regression wavelength caused by the vernier effect.